1. Field of the Invention
The present invention relates to color graphic display systems, and more particularly to a read/write control system for a buffer memory for use therein.
2. Background Art
In a conventional graphic display system adapted to display data as processed by a computer, instructions representing image features to be displayed are provided to a display processor which generates data for storage in a buffer having as many cells, or groups of cells, (hereinafter "storage locations") as there are picture elements (pixels) in the display unit. The storage locations store "attribute values", color look-up table addresses corresponding to the color values to be displayed at the corresponding monitor screen location. Such a buffer is frequently referred to as a frame buffer. The frame buffer is scanned in readout at the rate of scanning of the display device. The output is provided to the look-up table, and its output is provided to a digital-to-analog converter, the output of which, in turn, drives the display device itself.
In the absence of additional measures, new instructions to the display processor representing image features to be displayed at screen locations where image features are currently being displayed, result in the supplanting of frame buffer data at the cell locations corresponding to the new feature. In order words, the new image feature is painted over any previous features. This may, in fact, be appropriate for the image sequence being processed. However, in some cases it is desirable to have the new image feature appear to pass under previously existing image features. Alternatively, where a new image feature overlaps a previous image feature it may be desirable to have that area of overlap represented by a third color different from either of the colors of the two intersecting image features. These problems are often referred to in the art as "Underpaint" and "Line-on-Line", respectively.
At slow rates of image update, Line-on-Line and Underpaint have been provided. Current implementations involve software manipulation of the feature data to determine whether those conditions exist, and then generation of appropriate display instructions. However, as a practical matter the provision of a system which provides these capabilities at a sufficient rate to remain compatible with fast-scan large size display screens has heretofore eluded discovery.
Present schemes for Underpaint, for example, require the display processor to logically sort all image features spatially, farthest to nearest, and then send the image feature instructions to the control system for writing pixel information to the frame buffer in accordance with this sort. In other words, Underpaint is really a reverse overpaint effected via software manipulation. Such schemes are typically slow as compared with the scanning rate for the display device, resulting in a noticeable degradation in the smoothness and rapidity of the change of the image features on the display device.
Accordingly, it is desired to provide a color graphic display in which the color changes of portions of the display where image features intersect may be rapidly controlled in a manner which avoids degradation of the displayed image when it is in the process of change.